1. Field of the Invention
The present invention relates to a method and an apparatus therefor for performing image detail enhancement.
2. Description of the Prior Art
The term sharpness in the field of image processing refers to the appearance of sharp edges and contours in an image. The sharpness is one of the important characteristics which influence the image quality, especially, the pleasantness of the visual effect.
It is well known in the art that intensifying the image signal component having a spatial frequency of about 12 cpd (cycle per degree) is crucial to the enhancement of the sharpness. This is described in Kenji Hiwatashi, "Image Engineering Handbook," (Asakura Shoten, 1986), page 25, for example. The unit "cpd" denotes a number of pairs of black and white patterns within a visual angle of one degree. Therefore, an image signal component having the spatial frequency of about 12 cpd represents an image which includes about 12 pairs of black and white patterns within a visual angle of one degree.
The spatial frequency will be expressed herein in cpd or 1 p/mm (line pairs per millimeter). The unit "1 p/mm" denotes the number of pairs of black and white patterns within one millimeter on an image. Spatial frequency expressed in cpd, which is measured by an observer of an image, will be referred to as "apparent spatial frequency". Spatial frequency expressed in 1 p/mm, which is measured on an image, will be referred to as "actual spatial frequency".
In reproducing an image, a sharp and visually pleasing image is reproduced by intensifying the image signal component having the apparent spatial frequency of about 12 cpd.
A distance measured on an image which corresponds to a visual angle of one degree becomes longer as a distance between the image and an observer increases. Therefore the actual spatial frequency of the image signal component having the apparent spatial frequency of about 12 cpd decreases as the distance between the image and the observer increases.
Usually, the image signal component having the apparent spatial frequency of about 12 cpd is intensified while the distance between an image and an observer is assumed to be that of distinct vision, which usually ranges from 25 cm to 30 cm. However, values of parameters specifying the detail enhancement depend on an apparatus, such as a scanner, with which the detail enhancement is performed. For example, some apparatus varies the image signal component to be intensified in response to a magnification factor in reproduction. Further, the image signal component to be intensified depends on an operator's taste.
However, not all reproduced images are viewed at the distance of distinct vision. A reproduced image used as a frontispiece or an illustration of a comparatively small book is usually seen at the distance of distinct vision. On the other hand, an image used as a large-sized poster, for example, is usually seen at a distance longer than that of distinct vision, typically one meter or so away therefrom. If an image signal component has an apparent spatial frequency of about 12 cpd when measured at a distance of 25 cm, its apparent spatial frequency will be 48 cpd when measured at a distance of one meter. When this image signal component is intensified and the reproduced image is observed at the distance of one meter, the reproduced image does not seem highly sharp. This is also the case with an apparatus which varies the image signal component to be intensified in response to the magnification factor.
As described above, the conventional methods and apparatuses intensify image signal components having the aforementioned apparent spatial frequency of about 12 cpd under particular conditions, such as the distance of distinct vision. Consequently, the enhancement processing does not adequately enhance the sharpness of the image when the reproduced image is not seen at the distance of distinct vision.